The increasing use of portable communication devices with newer rechargable batteries has created a need for improved battery chargers. Examples of such communication devices are radiotelephones, radios, and modems. These newer nickel metal hydride (NiMH) batteries require very low, long-term overcharge rates to reduce the likelihood of overcharge damage to the battery. A typical charger charges at a high current rate until the battery is almost charged then reduces the charge rate to a low maintenance charge. This is typically done by monitoring the battery's temperature and reducing the charge rate as the battery temperature increases. When the battery temperature reaches a certain level, the charger changes to the lower, maintenance charge.
In order to eliminate the time that a communication device cannot be used due to charging, newer chargers allow the communication device to be connected to the charger to power the device while it is being used for communicating. This creates a problem if the battery is almost charged and the charger is in the maintenance charge mode. The communication device can require so much current that the charger in the maintenance mode cannot supply all the device's needs and the battery must be used to supply the rest. An example would be a radiotelephone in standby mode using 50 mA while the charger supplies only 25 mA. The radiotelephone user thinks the phone is charging when the battery is actually being drained. There is a resulting need for a process to charge a battery, without damaging the battery, while the device attached to the battery is being used.